[Saksham1105]

Summary

• Replace linear scan and prefix matching (mgr.list(limit=500) + s["id"].startswith(...)) in _session_detail() with exact-ID lookup via mgr.get(session_id).
• Eliminates prefix collision where requesting a prefix of another session ID could return the wrong session instead of a 404.
• Removes unnecessary table scans across 500 rows, performing a direct SQLite WHERE id = ? index lookup matching the design in fastapi_app.py.

Fixes #120

Testing

• Added regression test test_server_session_detail_exact_match_and_prefix_rejection in tests/test_phase6.py asserting:
  • Exact session ID lookup succeeds with status 200 and returns full details, events, and token totals.
  • Prefix lookups return 404 when no exact matching ID exists.
  • Non-existent IDs return 404.
• Executed ruff check devagent/ tests/ (all passed).
• Executed pytest across tests/test_api.py and tests/test_phase6.py (all 64 passed).